The invention relates to positioners which are widely used for a variety of applications including the positioning of optical components, both in production lines and in scientific experiments.
One shortcoming of positioners designed to provide a high degree of accuracy in their positioning is that they are often only able operate over a relatively limited range. For example, a number of positioner designs employ piezoelectric drive elements in their drive mechanisms. Piezoelectric drive elements are useful because they are relatively robust and are able to provide repeatable high precision positioning. However, a 38 mm long piezoelectric stack might have a positioning range of only 42 μm. This limited range can mean positioners employing piezoelectric drive elements can be impractical for many applications. Similar problems can arise with other types of high precision drive mechanisms.
U.S. Pat. No. 3,902,084, [1] and U.S. Pat. No. 3,902,085 [2] describe “inchworm positioners” which address this problem. These are one-axis positioners arranged to selectively grip and release opposing ends of a piezoelectric drive element as it is cyclically expanded and contracted. Appropriate control of the piezoelectric drive element and the piezoelectric grips at either end can cause the piezoelectric drive element to inch its way along between the grips during successive expand and contract cycles. Although positioners of this type allow for larger ranges of motion, they are complex designs requiring a number of moving parts which must be separately controlled in a carefully orchestrated manner.
Another approach is to use a positioner comprising two elements in frictional engagement with one another which slide relative to one another when the frictional force between them is overcome. Motion is generated by the interplay between inertia of one of the elements and slipping or sticking of the frictional engagement between the elements. Positioners of this kind are referred to as inertial positioners or slip-stick positioners and are known for example from an article by Dieter Pohl in the journal “Review of Scientific Instruments” [3]. Inertial slip-stick positioners can be made with relatively few major components. However, known inertial slip-stick positioners have a low load bearing capacity and a high degree of sensitivity to orientation, unlike the above described inchworm positioners.
An alternative solution would be to use a lever arrangement to magnify or amplify the motion. JP 02 119277 [4] describes a mechanical amplifier for a piezoelectric drive element, albeit one designed primarily for dot-matrix printer heads. The device of JP 02 119277 employs a pair of hinged levers mounted at opposing ends of a piezoelectric stack such that the lever's outer arms are brought together as their inner arms are pushed apart by the piezoelectric element expanding. A buckle spring connects between the lever's outer arms and buckles outwardly as the lever arms are brought together. The outward motion of the buckle spring provides the useful output of the device. EP 0 510 698 [5] employs principles similar to those of JP 02 119277, but has two buckle springs connected in series between the levers. This provides for two simultaneous displacement outputs. Devices of this kind are not generally suitable for the reliable positioning of sample platforms. This is because their sprung nature makes them prone to vibration, and their buckling movement will generally be different for different platform loads. This means samples having different masses will generally be positioned differently for the same expansion of the piezoelectric drive element.